Method for manufacturing a superconducting device having a thin superconducting channel formed of oxide superconductor material

ABSTRACT

A method of manufacturing a superconducting device involves forming a thin film on the surface of a substrate, forming a superconducting gate electrode on a portion of the thin film, etching the portions of the thin film using the gate electrode as a mask thereby providing a superconducting channel under the gate, forming a step portion on the superconducting channel and under the gate, converting the oxide portion of the step portion into a gate insulation portion by heating the substrate in a vacuum, forming a second oxide superconducting film on the exposed surface of the channel so that superconducting source and drain electrodes are formed on each side of the gate such that the drain and source have a thickness greater than that of the channel and are electrically isolated from the gate electrode.

This is a divisional of application Ser. No. 08/097,613, filed on Jul. 26, 1993, now U.S. Pat. No. 5,416,072, which is a continuation application Ser. No. 07/797,740, filed Nov. 1, 1991, (abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconducting device and a method for manufacturing the same, and more specifically to a superconducting device having a substantially planar upper surface and including a superconducting channel formed of oxide superconductor material, which channel, in the direction perpendicular to the substrate for the device, is relatively thin as compared to the thicknesses of the device's source and drain electrodes in that perpendicular direction.

2. Description of Related Art

Typical three-terminal devices which utilize a superconductor include a so called superconducting-base transistor and a so called super-FET (field effect transistor). The superconducting-base transistor includes an emitter of a superconductor or a normal conductor, a tunnel barrier of an insulator, a base of a superconductor, a semiconductor isolator and a collector of a normal conductor, stacked in the named order. This superconducting-base transistor operates at a high speed with a low power consumption, by utilizing high speed electrons passing through the tunnel barrier.

The super-FET includes a semiconductor layer, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on the semiconductor layer. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulator layer on the recessed or undercut rear surface of the portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode.

A superconducting current flows through the semiconductor layer portion between the superconductor source electrode and the superconductor drain electrode due to a superconducting proximity effect, and is controlled by an applied gate voltage. This super-FET also operates at a high speed with a low power consumption.

In addition, in the prior art, there has been proposed a three-terminal superconducting device having a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel.

Both of the above mentioned superconducting-base transistor and the super-FET have a portion in which a semiconductor layer and a superconducting layer are stacked to each other. However, it is difficult to form a stacked structure of the semiconductor layer and the superconducting layer formed of an oxide superconductor which has been recently advanced in study. In addition, even if it is possible to form a stacked structure of the semiconductor layer and the oxide superconducting layer, it is difficult to control a boundary between the semiconductor layer and the oxide superconducting layer. Therefore, a satisfactory operation could not been obtained in these superconducting devices.

In addition, since the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be located close to each other at a distance which is a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material, a distance between the superconductor source electrode and the superconductor drain electrode has to be not greater than a few ten nanometers. However, it is very difficult to conduct a fine processing such as a fine pattern etching so as to ensure the very short separation distance. Because of this, in the prior art, it has been impossible to manufacture the super-FET composed of the oxide superconductor material.

Furthermore, it has been confirmed that the conventional three-terminal superconducting device having the superconducting channel shows a modulation operation. However, the conventional three-terminal superconducting device having the superconducting channel could not realize a complete ON/OFF operation, because a carrier density is too high. In this connection, since an oxide superconductor material has a low carrier density, it is expected to form a three-terminal superconducting device which has a superconducting channel and which can realize the complete ON/OFF operation, by forming the superconducting channel of the oxide superconductor material. In this case, however, a thickness of the superconducting channel has to be made on the order of 5 nanometers.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an FET type superconducting device having a planar upper surface and including a superconducting region constituted of an extremely thin oxide superconductor film, which have overcome the above mentioned defects of the conventional ones.

Another object of the present invention is to provide a method for manufacturing an FET type superconducting device which have overcome the above mentioned defects of the conventional ones.

The above and other objects of the present invention are achieved in accordance with the present invention by a superconducting device comprising a substrate, a superconducting channel formed of an oxide superconductor on the principal surface of said substrate, a superconducting source electrode and a superconducting drain electrode, which are relatively thick as compared to the thickness of the channel, and are formed of the oxide superconductor on said superconducting channel separated from each other but electrically connected by said superconducting channel, so that a superconducting current can flow through said superconducting channel between the superconducting source electrode and the superconducting drain electrode, a superconducting gate electrode formed of the oxide superconductor through a gate insulator on said superconducting channel for controlling the superconducting current flowing through said superconducting channel, and a side insulating region formed to completely cover each of opposite side surfaces of the superconducting gate electrode, the superconducting channel and the gate insulator being formed of the same oxide thin film of which oxygen amount is locally decreased and said gate insulator being formed of a compound oxide which is composed of the same constituent elements as those of the oxide superconductor of which the superconducting channel is formed but includes the oxygen amount less than that of said oxide superconductor, and the superconducting gate electrode being surrounded by the superconducting channel, the superconducting source electrode and the superconducting drain electrode but electrically isolated from the superconducting channel, the superconducting source electrode and the superconducting drain electrode by the gate insulator and the side insulating region.

According to another aspect of the present invention, there is provided a method of manufacturing a superconducting device, comprising the steps of forming on a surface of a substrate a first oxide superconductor thin film, forming a superconducting gate electrode of the oxide superconductor on a portion of said oxide superconductor thin film, etching portions of said oxide superconductor thin film at the both sides of the superconducting gate electrode using the superconducting gate electrode as a mask, so that a superconducting channel of an extremely thin oxide superconductor thin film is formed under the superconducting gate electrode and a step portion is formed on an upper surface of the superconducting channel and under the superconducting gate electrode, heating the substrate under high vacuum so that oxygen of the oxide superconductor of the step portion escapes and the oxide superconductor of the step portion is converted into an oxide insulating material and the step portion changes to a gate insulator, and growing a second oxide superconductor thin film on the exposed surface of the superconducting channel, using the gate electrode as a mask, so that a superconducting source electrode and a superconducting drain electrode having a relatively thick thickness are formed at opposite sides of the superconducting gate electrode, electrically isolated from the gate electrode.

In a preferred embodiment, the oxide superconductor is formed of high-T_(c) (high critical temperature) oxide superconductor, particularly, formed of a high-T_(c) copper-oxide type compound oxide superconductor for example a Y-Ba-Cu-O type compound oxide superconductor material, a Bi-Sr-Ca-Cu-O type compound oxide superconductor material, and a TI-Ba-Ca-Cu-O type compound oxide superconductor material.

In addition, the substrate can be formed of an insulating substrate, preferably an oxide single crystalline substrate such as MgO, SrTiO₃, CdNdAlO₄, etc. These substrate materials are very effective in forming or growing a crystalline film having a high orientation property. However, the superconducting device can be formed on a semiconductor substrate if an appropriate buffer layer is deposited thereon. For example, the buffer layer on the semiconductor substrate can be formed of a double-layer coating formed of a MgAlO₄ layer and a BaTiO₃ layer if silicon is used as a substrate.

Preferably, the superconducting channel is formed of a c-axis orientated oxide superconductor thin film and the superconducting source electrode and the superconducting drain electrode is formed of a-axis orientated oxide superconductor thin films.

In a preferred embodiment, the superconducting channel, the gate insulator and the superconducting gate electrode are formed of the same oxide thin film of which oxygen amount is locally decreased and said gate insulator being formed of a compound oxide which is composed of the same constituent elements as those of the oxide superconductor of which the superconducting channel and the superconducting gate electrode are formed but includes the oxygen amount less than that of said oxide superconductor.

In this preferred embodiment, the superconducting channel and the superconducting gate electrode are formed of a c-axis orientated oxide superconductor thin film.

In another preferred embodiment, the superconducting gate electrode are formed of a a-axis orientated oxide superconductor thin film.

In the superconducting device manufactured in accordance with the present invention, the superconducting current flowing between the superconducting source electrode and the superconducting drain electrode through the superconducting channel is controlled by a voltage applied to the superconducting gate electrode. Namely, the superconducting device constitutes the super-FET. In order to ensure that the superconducting channel can be turned on and off by a voltage applied to the superconducting gate electrode, the superconducting channel has to be made extremely thin, on the order of 5 nanometers in the direction of an electric field created by the voltage applied to the superconducting gate electrode. This extremely thin superconducting channel can be easily realized or formed and the superconducting gate electrode, the gate insulator and the superconducting channel are arranged in a self alignment in accordance with the method of the present invention.

In a preferred embodiment, a first oxide superconductor thin film is formed to have the thickness on the order of about 300 nanometers. The first oxide superconductor thin film is selectively etched by a reactive ion etching process or an ion milling using Ar-ions, so that a portion of the oxide superconductor thin film projects and the portion becomes a superconducting gate electrode. Forming an insulating member which covers the side surface of the superconducting gate electrode, the first oxide superconductor thin film excluding the superconducting gate electrode is further etched so that a superconducting channel having a thickness of less than 5 nanometers is formed under the superconducting gate electrode. Simultaneously, a step portion having a thickness of not less than 10 nanometers, which will be a gate insulator when the super-FET will be completed, is formed on a surface of the superconducting channel and under the superconducting gate electrode. To convert the step portion into the oxide insulating layer, the substrate is heated under high vacuum.

Oxygen of crystals of an oxide superconductor escapes when the oxide superconductor is heated under vacuum. Superconducting properties of an oxide superconductor is sensitive to amounts of oxygen which is included in the crystals of the oxide superconductor. If the crystals of the oxide superconductor lack oxygen, the critical temperature of the oxide superconductor lowers considerably or the oxide superconductor loses its superconductivity.

Therefore, the step portion is converted into an oxide insulating layer, under which the extremely thin oxide superconductor thin film can be formed. The thickness of the oxide insulating layer is controlled by the heat process time. It is preferable to etch the oxide superconductor film so that side surfaces of a portion which will be converted into the oxide insulating layer, which are parallel to the c-axes of crystals of oxide superconductor, are exposed, since the oxygen of crystals of oxide superconductor migrates to a direction perpendicular to the c-axis of crystals of oxide superconductor easier than directions parallel to the a-axes and b-axes of crystals of oxide superconductor.

In another preferred embodiment, a first oxide superconductor thin film is formed to have the thickness on the order of about 20 nanometers and an a-axis orientated oxide superconductor thin film is formed on the first oxide superconductor thin film to have the thickness on the order of about 200 nanometers. A surface protector is deposited on the a-axis orientated oxide superconductor thin film at a position of a superconducting gate electrode to be formed in future. An exposed portion of the a-axis orientated oxide superconductor thin film and a portion of the first oxide superconductor thin film under the a exposed portion of the a-axis orientated oxide superconductor thin film are etched by means of an anisotropic etching so as to form a superconducting gate electrode, a step portion and a superconducting channel. For this purpose, the a-axis orientated oxide superconductor thin film is selectively etched so as to remove all of the a-axis orientated oxide superconductor thin film excluding a portion which becomes the superconducting gate electrode on the superconducting channel, so that the superconducting gate electrode is formed. Then, the first oxide superconductor thin film is selectively etched in a self alignment to the patterned superconducting gate electrode, so that a step portion is left on the superconducting channel and only under the patterned superconducting gate electrode. Thereafter, a heat-treatment mentioned above is carried out. In this embodiment, the upper surface of the superconducting gate electrode formed of a a-axis orientated oxide superconductor thin film is are covered with the surface protector, completely. Therefore, little oxygen of the superconducting gate electrode escapes during the heat-treatment, so that the superconductivity of the superconducting gate electrode is maintained.

The extremely thin oxide superconductor film thus formed is very preferable in thickness and the crystal orientation to form a superconducting channel. In addition, according to the present invention, the superconducting gate electrode, the gate insulator and the superconducting channel are arranged in a self alignment.

As seen from the above, the method in accordance with the present invention includes no process which requires high-precision for forming the superconducting channel. Therefore, the limitation in the fine processing techniques required for manufacturing the super-FET can be relaxed.

The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H and 1I are diagrammatic sectional views for illustrating a first embodiment of the process in accordance with the present invention for manufacturing tile super-FET;

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G and 2H are diagrammatic sectional views for illustrating a second embodiment of the process in accordance with the present invention for manufacturing the super-FET.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1A to 1I, the process in accordance with the present invention for manufacturing the super-FET will be described.

As shown in FIG. 1A, a substrate 5 having a substantially planer principal surface is prepared. This substrate 5 is formed of for example, an insulator substrate such as a MgO (100) substrate, a SrTiO₃ (100) substrate, a CdNdAlO₄ (001) substrate or others, or a semiconductor substrate such as a silicon substrate having a principal surface coated with a buffer layer composed of an insulating film. In the case of the silicon substrate, the principal surface of the silicon is preferably coated with MgAlO₄ by a CVD (chemical vapor deposition) and also with BaTiO₃ by a sequential sputtering process.

As shown in FIG. 1B, a c-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film I having a thickness on the order of about 300 nanometers is deposited on the principal surface of the substrate 5, by for example an off-axis sputtering, a reactive evaporation, an MBE (molecular beam epitaxy), a CVD. etc. A lower portion of this oxide superconductor thin film 1 forms a superconducting channel 10 when the super-FET is completed. The superconducting channel is preferably formed of c-axis orientated thin film, since the c-axis orientated thin film has a large critical current density in the direction in parallel to the substrate surface. A condition of forming the c-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film 1 by off-axis sputtering is as follows:

    ______________________________________                                         Sputtering Gas          Ar: 90%                                                                        O.sub.2 : 10%                                          Pressure                10 Pa                                                  Temperature of the substrate                                                                           700° C.                                         ______________________________________                                    

The oxide superconductor thin film is preferably formed of a high-T_(c) (high critical temperature) oxide superconductor material, particularly a high-T_(c) copper-oxide type compound oxide superconductor material, for example, a Bi-Sr-Ca-Cu-O type compound oxide superconductor material, or a TI-Ba-Ca-Cu-O type compound oxide superconductor material other than Y-Ba-Cu-O type compound oxide superconductor material.

Then, as shown in FIG. 1C, the oxide superconductor thin film 1 is etched by means of an anisotropic etching so as to form a superconducting gate electrode 4. For this purpose, the oxide superconductor thin film 1 is selectively etched by a reactive ion etching process or an ion milling using Ar-ions, so that the oxide superconductor thin film 1 excluding a portion which will be a superconducting gate electrode has a thickness of on the order of 20 nanometers.

Thereafter, as shown in FIG. 1D, a insulating layer 8 formed of for example MgO is deposited on the oxide superconductor thin film 1 and the superconducting gate electrode 4. The insulating layer 8 is etched by means of an anisotropic etching so as to form insulating members 18 which surround the superconducting gate electrode 4, as shown in FIG. 1E. This etching process is also carried out by a reactive ion etching process or an ion milling using Ar-ions.

Then, as shown in FIG. 1F, the oxide superconductor thin film 1 excluding the superconducting gate electrode 4 is etched by means of an isotoropic etching so that a superconducting channel 10 is formed under the superconducting gate electrode 4 and a step portion 16 is formed on a surface of the superconducting channel 10 and under the superconducting gate electrode 4. The step portion 16 will be a gate insulator, when the super-FET will be completed. The step portion 16 is formed having a thickness of not less than 10 nanometers and the superconducting channel 10 is formed having an extremely thin thickness of less than 5 nanometers by a reactive ion etching process or an ion milling using Ar-ions. In this connection, it is desired that the step portion 16 is side-etched in comparison with the superconducting gate electrode 4, so that the step portion 16 has a length shorter than that of the superconducting gate electrode 4.

It is desired that the condition of the oxide superconductor thin film 1 is monitored, while the oxide superconductor thin film 1 is etched to form the superconducting channel 10, so as to prevent the superconducting channel 10 from containing a degraded portion of the oxide superconductor thin film 1.

Thereafter, the substrate 5 is heated to more than 400° C. under a pressure of 1×10⁻⁵ Pa. Then, as shown in FIG. 1G, oxygen of the crystals of the oxide superconductor of the step portion 16 escapes through side surfaces so that the step portion 16 changes to the gate insulator 6 and the portion of the superconducting channel 10 under the gate insulator 6 have a thickness of less than 5 nanometers. In this process the oxygen of the crystals of the oxide superconductor of the step portion escapes only through side surfaces, since the oxide superconductor has larger diffusion coefficients of oxygen along the a-axis and the b-axis of the crystal than along the c-axis.

Then, as shown in FIG. 1H, an insulating layer 17 which surrounds the insulating members 18 and the gate insulator 6 is formed of an oxygen barrier material such as a silicon nitride by a sputtering process etc. A cavity may be created beside the gate insulator 6. The insulating layer 17 and the insulating members 18 are integrated into an insulating layer 7.

Finally, as shown in FIG. 1I, a superconducting source electrode 2 and a superconducting drain electrode 3 of an a-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film are formed on the superconducting channel 10 at both sides of the superconducting gate electrode 4 and of gate insulator 6, respectively. An a-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film can be formed by an off-axis sputtering process which is carried out under a condition that the substrate temperature is not higher than 650° C. A sputtering condition under which the superconducting source electrode 2 and the superconducting drain electrode 3 are formed is as follows:

    ______________________________________                                         Sputtering Gas          Ar: 90%                                                                        O2: 10%                                                Pressure                10 Pa                                                  Temperature of the substrate                                                                           640° C.                                         ______________________________________                                    

In the process, in order to planarize an upper surface of the deposited a-axis orientated oxide superconductor thin film and to shape the superconducting source electrode 2 and the superconducting drain electrode 3, a photoresist layer (not shown) is deposited on the a-axis orientated oxide superconductor thin film in such a manner that the deposited photoresist layer has a flat upper surface, and then, the deposited photoresist layer and the deposited a-axis orientated oxide superconductor thin film are etched back, until the upper surface of the a-axis orientated oxide superconductor thin film is planarized and the superconducting gate electrode 4 is exposed at the planarized upper surface of the a-axis orientated oxide superconductor thin film as shown in FIG. 1I. A metal source electrode and a metal drain electrode may be formed on the superconducting source electrode 2 and the superconducting drain electrode 3 respectively, if necessary. With this, the super-FET in accordance with the present invention is completed.

As explained above, if the super-FET is manufactured in accordance with the first embodiment of the method of the present invention, the limitation in the fine processing technique required for manufacturing the super-FET is relaxed. Since the flatness of the upper surface of the superconducting device can be improved, it become easy to form conductor wirings in a later process. Further more, since the gate insulator is formed of an oxide which is made of the same composition and has same crystal structure as those of the oxide superconductor constituting the superconducting gate electrode and the superconducting channel, the super-FET is superior in mechanical stability and interfaces between the gate insulator and the superconducting gate electrode, and between the gate insulator and the superconducting channel are formed sharp. Accordingly, it is easy to manufacture the super-FET with good repeatability, and the manufactured super-FET has a stable performance.

Referring to FIGS. 2A to 2H, a second embodiment of the process for manufacturing the superconducting device will be described.

As shown in FIG. 2A, there is prepared a substrate 5, similar to the substrate 5 shown in FIG. 1A.

As shown in FIG. 2B, a c-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film 1 having a thickness on the order of about 20 nanometers is deposited on the principal surface of the substrate 5, by for example an off-axis sputtering. The off-axis sputtering is performed under the same condition as that of the first embodiment. A lower portion of this oxide superconductor thin film 1 forms a superconducting channel 10 when the super-FET is completed.

As shown in FIG. 2C, an a-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film 11 having a thickness on the order of about 200 nanometers is deposited on the c-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film 1. The oxide superconductor thin film 11 will be shaped into a superconducting gate electrode. The off-axis sputtering is performed under the same condition under which the superconducting source electrode 2 and the superconducting drain electrode 3 are formed in the first embodiment.

Then, as shown in FIG. 2D, a surface protector 9 is deposited on the oxide superconductor thin film 11 at a position of a superconducting gate electrode to be formed in future. Preferably, the surface protector 9 is formed of a metal such as Au, or an insulating material and it is desired that the surface protector 9 is constituted by stacked layers including a metal layer or a insulator layer and photo resist film, if necessary.

Thereafter, an exposed portion of the oxide superconductor thin film 11 and a portion of the oxide superconductor thin film 1 under the a exposed portion of the oxide superconductor thin film 11 are etched by means of an anisotropic etching so as to form a superconducting gate electrode 4, a step portion 16 and a superconducting channel 10, as shown in FIG. 2E. For this purpose, the oxide superconductor thin film 11 is selectively etched by a reactive ion etching process or an ion milling using Ar-ions so as to remove all of the oxide superconductor thin film 11 excluding a portion which becomes the superconducting gate electrode on the superconducting channel 10, so that the superconducting gate electrode 4 is formed. Then, the oxide superconductor thin film 1 is selectively etched in a self alignment to the patterned superconducting gate electrode 4, so that a step portion 16 is left on the superconducting channel 10 and only under the patterned superconducting gate electrode 4. The step portion 16 will be a gate insulator, when the super-FET will be completed. The step portion 16 is formed having a thickness of not less than 10 nanometers and the superconducting channel 10 is formed having a thickness of less than 5 nanometers. In this connection, it is desired that the step portion 16 is side-etched in comparison with the superconducting gate electrode 4, so that the step portion 16 has a length shorter than that of the superconducting gate electrode 4, if necessary.

It is desired that the condition of the oxide superconductor thin film 1 is monitored, while the oxide superconductor thin film 1 is etched to form the superconducting channel 10, so as to prevent the superconducting channel 10 from containing a degraded portion of the oxide superconductor thin film 1.

Thereafter, the substrate 5 is heated to more than 400° C. under a pressure of 10⁻⁵ Pa. Then, as shown in FIG. 2F, oxygen of the crystals of the oxide superconductor of the step portion 16 escapes through side surfaces so that the step portion 16 changes to the gate insulator 6 and the portion of the superconducting channel 10 under the gate insulator 6 have a thickness of less than 5 nanometers. In this process the oxygen of the crystals of the oxide superconductor of the step portion escapes only through side surfaces, since the oxide superconductor has larger diffusion coefficients of oxygen along the a-axis and the b-axis of the crystal than along the c-axis. Particularly, since the superconducting gate electrode 4 is formed of an a-axis orientated oxide superconductor thin film and the surface protector is formed on the upper surface of the superconducting gate electrode 4, little oxygen escapes from the superconducting gate electrode 4.

Thereafter, as shown in FIG. 2G, a insulating layer 7 which surrounds the surface protector 9, the superconducting gate electrode 4 and the gate insulator 6 is formed of an oxygen barrier material such as a silicon nitride.

Finally, as shown in FIG. 2H, a superconducting source electrode 2 and a superconducting drain electrode 3 of an a-axis orientated Y₁ Ba₂ Cu₃ O₇₋δ oxide superconductor thin film are formed on the superconducting channel 10 at both sides of the superconducting gate electrode 4 and of gate insulator 6, respectively by for example an off-axis sputtering. The off-axis sputtering is performed under the same condition as that of the first embodiment. A metal source electrode and a metal drain electrode may be formed on the superconducting source electrode 2 and the superconducting drain electrode 3 respectively, if necessary.

As explained above, if the above mentioned super-FET is manufactured in accordance with the above mentioned process, the limitation in the fine processing technique required for manufacturing the super-FET is relaxed. In addition, since the substantially planarized upper surface is obtained, it become easy to form conductor wirings in a later process. Further more, since the gate insulator is formed of an oxide which is made of the same composition and has same crystal structure as those of the oxide superconductor constituting the superconducting gate electrode and the superconducting channel, the super-FET is superior in mechanical stability and interfaces between the gate insulator and the superconducting gate electrode, and between the gate insulator and the superconducting channel are formed sharp. Accordingly, it is easy to manufacture the super-FET with good repeatability, and the manufactured super-FET has a stable performance.

In the above mentioned embodiments, the oxide superconductor thin film can be formed of not only the Y-Ba-Cu-O type compound oxide superconductor material, but also a high-T_(c) (high critical temperature) oxide superconductor material, particularly a high-T_(c) copper-oxide type compound oxide superconductor material, for example a Bi-Sr-Ca-Cu-O type compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O type compound oxide superconductor material.

The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but converts and modifications may be made within the scope of the appended claims. 

We claim:
 1. A method of manufacturing a superconducting device, comprising the steps of:forming a first oxide superconductor thin film on a surface of a substrate, forming a superconducting gate electrode of the oxide superconductor on a portion of said oxide superconductor thin film, etching portions of said oxide superconductor thin film at sides of the superconducting gate electrode using the superconducting gate electrode as a mask, so that a superconducting channel of an oxide superconductor thin film having a thickness in a direction perpendicular to said substrate surface is formed under the superconducting gate electrode, forming a step portion on an upper surface of the superconducting channel and under the superconducting gate electrode, heating the substrate under high vacuum so that oxygen of the oxide superconductor of the step portion escapes and the oxide superconductor of the step portion is converted into an oxide insulating material and the step portion is changed to a gate insulator, and growing a second oxide superconductor thin film on the exposed surface of the superconducting channel by using the gate electrode as a mask, so that a superconducting source electrode and a superconducting drain electrode, each having a thickness greater than the thickness of the channel in said direction, are formed at opposite sides of the superconducting gate electrode and electrically isolated from the gate electrode.
 2. A method claimed in claim 1 wherein the first oxide superconductor thin film is a c-axis orientated oxide superconductor thin film and the second oxide superconductor thin film is an a-axis orientated oxide superconductor thin film.
 3. A method claimed in claim 2 wherein the superconducting gate electrode, the gate insulator and the superconducting channel are formed of the same oxide thin film which is prepared from the first oxide superconductor thin film.
 4. A method claimed in claim 1 further comprising the steps of depositing an a-axis orientated oxide superconductor thin film on the first oxide superconductor thin film for a superconducting gate electrode, forming a surface protector on a portion of said superconducting gate electrode, and etching portions of said a-axis orientated oxide superconductor thin film at sides of the surface protector so as to remove all of the a-axis orientated oxide superconductor thin film, excluding a portion which becomes the superconducting gate electrode, using the surface protector as a mask. 